My knees have become terrible over time, serious pain if i walk even 20 minutes. what should i do?

Seek evaluation: at your young age, it is unusual for the knees to be that painful. something is likely causing the pain, so evaluation is warranted. examination, xrays are a place to start. an MRI may be warranted depending on the prior findings.

You need an orthopaedic evaluation. Start with appointment with Orthopaedic surgeon. They will get x-rays and examine you. If indicated, an MRI will be ordered next. Could be meniscus tears or early arthritis from an old injury or systemic inflammatory condition.